1. Field of the Invention
The present invention relates to optical ring network management, including a method, apparatus, and optical add drop multiplexing node thereof.
2. Description of the Related Art
Recently, with the explosively expanding demand for data communication indicated by the Internet traffic, increases in the capacity of each network are desired. In addition, networks are desired to be highly flexible and economical because the services provided therethrough are getting more diversified. Especially, networks employing optical transmission (hereinafter, “optical network”) are the core for forming the basis of information communication networks and the development thereof is rapidly progressing because faster speeds and a wider areas for providing services are further desired.
For optical networks, the WDM technique of simultaneously transmitting multiple signals using one optical fiber by multiplexing light beams having different wavelengths is widely used. At a node that executes transmission using wavelength division multiplexing (WDM), a multiplexed optical signal (hereinafter, “WDM signal”) is processed for each wavelength. Therefore, the WDM signal is controlled by an optical add drop multiplexer (OADM) that executes adding and dropping of an optical signal having a specific wavelength without converting the WDM signal into an electric signal. An approach to realize the function of the OADM can include a method of using a wavelength-tunable filter that selects and transmits therethrough an optical signal having a desired wavelength from a WDM signal (see, for example, Japanese Patent Application Laid-Open Publication No. 2004-235741).
Conventionally, a form that is often employed in a metropolitan network of various forms for optical networks is an optical ring network as shown in FIG. 9. FIG. 9 depicts an optical ring network employing WDM. An optical ring network B includes a ring line 10 and four OADM nodes 9 (Node 1 to Node 4) that are installed on the ring line 10, each having an OADM function. FIG. 10 depicts an OADM node disposed on the optical network. The OADM node 9 includes an optical coupler 11 of one by two (having one input and two outputs) into which a WDM signal is input, a wavelength tunable filter unit 12 connected to an end of the optical coupler 11, and a rejection adding filter 13 connected to the other end of the optical coupler 11.
The wavelength tunable filter unit 12 can drop an optical signal having an arbitrary wavelength of the WDM signal alone by using an acousto-optic tunable filter (AOTF). The configuration of the wavelength tunable filter unit 12 differs depending on the device employed therein as a wavelength tunable filter. However, when an AOTF is used, the unit 12 is configured by an AOTF, an RF signal unit, a control unit, etc., due to the AOTF characteristic of being able to transmit an optical signal having an arbitrary wavelength by the variation of a radio frequency (RF) applied thereto as a control signal.
The input side of the rejection adding filter 13 is connected to the optical coupler 11 and a line 14 that inputs a specific wavelength λ1 from the outside. The output side of the rejection adding filter 13 is connected to the ring line 10. The rejection adding filter 13 adds the optical signal having the specific wavelength from the line 14. The optical signal is coupled with another optical signal input from the optical coupler 11 and, thereby, becomes a WDM signal, and is output to the ring line 10. The reject function is a function to, when an added optical signal travels round once in the optical ring network B and returns to the OADM node 9 where the optical signal is added, e.g., from Node 1 to Node 1, terminate only the optical signal having the specific wavelength λ1 in the WDM signal input from the optical coupler 11 to prevent the optical signal from being multiplexed with other optical signals having the same wavelength added from the line 14. The wavelength of an optical signal to be added is assigned in advance to each node and the selection of a wavelength to be received at the wavelength tunable filter unit 12 is the setting of a communication counterpart. Since a wavelength to be received can arbitrarily set at the wavelength tunable filter unit 12, communication between arbitrary nodes is enabled.
Based on the configuration of the optical ring network B, another optical ring network has also been proposed that includes two optical fibers as a measure in the case of a malfunction in the network such as a disconnection of an optical fiber. FIG. 11 depicts an optical ring network using two optical fibers. FIG. 12 depicts a table of multiplexed states of the WDM signal at each node in the network depicted in FIG. 11. An optical ring network C is configured by an active line 20, a backup line 30 for the active line 20, and four OADM nodes 19 (Node 1 to Node 4). As depicted in FIG. 12, the same WDM signal is transmitted to any of the OADM nodes 19 (Node 1 to Node 4) on the active line 20 and the backup line 30 and, therefore, even when the active line 20 is disconnected, an instant recovery of the communication is enabled by executing protection of the active line 20 using an optical unidirectional path switched ring (O-UPSR) that instantly switches the connection at a node to the backup line 30 (see, for example, Japanese Patent Application Laid-Open Publication No. 2001-156821).
Though instant recovery is possible even when the two optical fibers are simultaneously disconnected, no setting can be made for a new wavelength path in the conventional network. FIG. 13 depicts an example of simultaneous disconnections of optical fibers of an existing line and a backup line in an optical ring network that uses two optical fibers. FIG. 14 depicts a table of multiplexed states of the WDM signal at Node 1 to Node 4 in the network configuration depicted in FIG. 13.
On the active line 20, at the time to execute dropping, the WDM signal can not reach the OADM node 19 (Node 1) immediately after a disconnected point 40 on the optical fibers and because an optical signal having a specific wavelength λ1 is added at Node 1, a WDM signal multiplexed with only the optical signal having the wavelength λ1 is transmitted to Node 2 at the time to execute the dropping. Similarly, an optical signal having a wavelength λ2 is added at Node 2, an optical signal having a wavelength λ3 is added at Node 3, and an optical signal having a wavelength λ4 is added at Node 4. Therefore, the number of signals that are multiplexed on the WDM signal is the lowest at a point immediately after the disconnected point 40, and the number of signals that are multiplexed on the WDM signal increases in the direction of the transmission on the active line 20 at each of the OADM nodes 19. Therefore, the number of signals that are multiplexed on the WDM signal is largest at Node 4 immediately before the disconnected point 40.
On the backup line 30, the WDM signal does not reach Node 4, which is positioned immediately after the disconnected point 40 in the transmitting direction of the backup line 30. Similar to the above multiplexed state of the WDM signal at each of the OADM nodes 19 on the active line 20, the number of signals that are multiplexed on the WDM signal increases at each of the OADM nodes 19 in the transmitting direction in the order of Node 4→Node 3→Node 2→Node 1.
As described above, it is impossible for the optical signal λ4 added at Node 4 to be transmitted to Node 1 on the active line 20 and for the optical signal λ1 added at Node 1 to be transmitted to Node 3 on the backup line 30. The selection of a transmission line is limited to either the active line 20 or the backup line 30 and depending on the position of the disconnected point 40, the multiplexed states of the WDM signal to be transmitted varies depending on whether the active line or the backup line is selected. Therefore, no setting for a new wavelength path can be made.
To set a new wavelength path when the two optical fibers are disconnected simultaneously, the disconnected positions of the optical fibers must be known and the OADM nodes between which communication is possible must be judged, and thereafter, the optical signal wavelength to be added that has been defined at each of the OADM nodes must be reset. To do this, operating wavelength information recorded at each of the OADM nodes must be updated. Updating of the operating wavelength information can be realized by adding an optical spectrum monitor to each of the OADM nodes. However, optical spectrum monitors are generally expensive making the addition of an optical spectrum monitor to each of the OADM nodes impractical.